5

我想在我的 WPF 应用程序中显示用户 Gravatar。这就是我绑定图像控件的方式：

<Image Source="{Binding Path=Email, Converter={StaticResource GravatarConverter},IsAsync=True}">

GravatarConverter 返回给定电子邮件的 URL。不幸的是，这在加载第一个图像时完全阻止了我的 UI。请注意，我使用的是“IsAsync=True”。经过一些研究，我发现在应用程序启动时在单独的线程中调用 FindServicePoint 时可以解决这个问题：

        Task.Factory.StartNew( () => ServicePointManager.FindServicePoint( "http://www.gravatar.com", WebRequest.DefaultWebProxy ) );

但是，当我的应用程序已经在下载图像时 FindServicePoint 没有完成时，这不起作用。有人可以解释一下为什么 WPF-App 需要这个 FindServicePoint，为什么会阻塞 UI 以及如何避免阻塞？

谢谢

更新：事实证明，在我取消选中 Internet Explorer 的“Internet 选项”->“连接”->“LAN 设置”中的“自动检测设置”后，我的问题就消失了。

我使用这个非常简单的 WPF 应用程序来重现问题，只需在文本框中插入图像的 url 并单击按钮。启用“自动检测设置”后，应用程序会在第一次加载图像时冻结几秒钟。使用此选项立即禁用其加载。

主窗口.xaml

<Window x:Class="WpfGravatarFreezeTest.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>
    <TextBox Grid.Column="0" Grid.Row="0" HorizontalAlignment="Stretch" x:Name="tbEmail" />
    <Button Grid.Column="0" Grid.Row="0" Click="buttonLoad_OnClick" HorizontalAlignment="Right">Set Source</Button>
    <Image x:Name="img" Grid.Column="0" Grid.Row="1" Grid.ColumnSpan="2" />
</Grid>

主窗口.xaml.cs

using System;
using System.Windows;
using System.Windows.Media.Imaging;

namespace WpfGravatarFreezeTest
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void buttonLoad_OnClick( object sender, RoutedEventArgs e )
        {
            try { this.img.Source = new BitmapImage(new Uri(this.tbEmail.Text)); }
            catch( Exception ){}            
        }
    }   
}
4

1 回答 1

2

Blocking UI happans because IsAsync=True runs in async manner only binding process. In your case you have a long running operation during converting process. To solve this you should create converter that presents the result asynchronously like this (based on this answer):

Create task complition notifier:

public sealed class TaskCompletionNotifier<TResult> : INotifyPropertyChanged
{
    public TaskCompletionNotifier(Task<TResult> task)
    {
        Task = task;
        if (task.IsCompleted) return;
        task.ContinueWith(t =>
        {
            var temp = PropertyChanged;
            if (temp != null)
            {
                temp(this, new PropertyChangedEventArgs("Result"));
            }
        }); 
    }

    // Gets the task being watched. This property never changes and is never <c>null</c>.
    public Task<TResult> Task { get; private set; }

    // Gets the result of the task. Returns the default value of TResult if the task has not completed successfully.
    public TResult Result { get { return (Task.Status == TaskStatus.RanToCompletion) ? Task.Result : default(TResult); } }

    public event PropertyChangedEventHandler PropertyChanged;

}

Create async converter implementing MarkupExtention:

public class ImageConverter: MarkupExtension, IValueConverter
{

    public ImageConverter()
    {
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null) return new BitmapImage();
        var task = Task.Run(() =>
        {
            Thread.Sleep(5000); // Perform your long running operation and request here
            return value.ToString();
        });

        return new TaskCompletionNotifier<string>(task);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }

}

Use it in Xaml:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition/>
    </Grid.RowDefinitions>

    <TextBox x:Name="uri" Grid.Row="0" Text="{Binding ImageUri, ElementName=main}"/>
    <Image Grid.Row="1" DataContext="{Binding Text, ElementName=uri, Converter={local:ImageConverter}}" Source="{Binding Path=Result, IsAsync=True}"/>

</Grid>

Update 2 Seems like Image control load images asynchronously itself. You are right first load take a lot of time. You may use code like this:

    try
    {
        var uri = Uri.Text;
        var client = new WebClient();
        var stream = await client.OpenReadTaskAsync(uri);
        var source = new BitmapImage();
        source.BeginInit();
        source.StreamSource = stream;
        source.EndInit();
        Img.Source = source;


    }
    catch (Exception) { }

But its performance is't better than your variant.

于 2013-10-15T11:00:19.600 回答